The present invention relates to a method of producing an electrooptical device and also to a method of producing a driving substrate for such an electrooptical device, the method being particularly suitable for production of, for example, a liquid crystal display device having an active region and a passive region, wherein the active region including a thin-film insulating-gate field-effect transistor of dual-gate type (hereinafter referred to as dual-gate MOSTFT)or of a bottom-gate type (hereinafter referred to as bottom-gate MOSTFT) using a single-crystal silicon layer grown by graphoepitaxy on an insulating substrate. The bottom-gate MOSTFT includes both an inverse-stagger NSI type and an inverse stagger ISI type MOSTFTs.
Various types of active-matrix liquid crystal displays are known: for example, a liquid crystal display having a display section using amorphous silicon for TFTs and an IC for an external driving circuit; a liquid crystal display integrating a driving circuit and a display section using solid-phase-deposited polycrystalline silicon for TFTs, as disclosed in Japanese Patent Application Laid-Open No. 6-242433); and a liquid crystal display device integrating a driving circuit and a display section using excimer laser annealing polycrystalline silicon TFTs, as disclosed in Japanese Patent Application Laid-Open No. 7-131030.
Although these known amorphous silicon TFTs have high productivity, they are not suitable for production of p-channel MOSTFTs (hereinafter referred to as pMOSTFTs) due to a low electron mobility of 0.5 to 1.0 cm2/vxc2x7sec. Since a peripheral driving section using pMOSTFTs and a display section cannot be formed on the same substrate, the driver IC must be an external component, which is mounted by, for example, a tape automated bonding (TAB) method, causing an impediment to reduction of the cost. This configuration inhibits production of high-resolution devices. Furthermore, the electron mobility as small as 0.5 to 1.0 cm2/vxc2x7sec can produce only a small ON current; hence, the size of the transistors in the display section is inevitably large, resulting in a small aperture ratio of pixels.
Conventional polycrystalline silicon TFTs have an electron mobility of 70 to 100 cm2/vxc2x7sec and can facilitate production of high-resolution devices, so that liquid crystal displays (LCDs) which use polycrystalline silicon and which are integrated with driving circuits are becoming conspicuous. The above electron mobility, however, is insufficient for driving a large LCD of 15 inches or more, and thus ICs for an external driving circuit are still required.
TFTs using polycrystalline silicon formed by a solid-phase deposition process require annealing at a temperature of 600xc2x0 C. or more for ten or more hours and thermal oxidation at approximately 1,000xc2x0 C. to form a gate SiO2 layer, necessitating the-use of a semiconductor production apparatus. Thus, the wafer size is limited to 8 to 12 inches and the use of expensive heat-resistant quartz glass is inevitable, causing an impediment to reduction in the cost. Thus, the use of such TFTs is limited-to EVF and audiovisual (AV) projectors.
Polycrystalline silicon TFTs produced by excimer laser annealing have many problems, including unstable output of the excimer lasers, low productivity, increasing price of the apparatus with increasing size, low yield and low quality.
These problems are pronounced when large glass substrates having a side length of, for example, 1 meter are used.
It is an object of the present invention is to make it possible to produce an active matrix substrate incorporating a high-performance driver, as well as an electrooptical device which is typically a display thin-film semiconductor device using such an active matrix substrate, through a uniform deposition of a single-crystal silicon layer having high electron/hole mobility particularly in a peripheral-driving-circuit section.
It is also an object of the present invention to implement a structure in which a display section and a peripheral-driving-circuit portion are integrated, wherein the display section comprises an n-channel MOSTFT (referred to as xe2x80x9cnMOSTFTxe2x80x9d, hereinafter) or a pMOSTFTs employing an LDD (Lightly Doped Drain) structure having high switching characteristic and operable with reduced leak current, or a complementary insulating gate field effect transistor (referred to as cMOSTFT) having high driving performance, while the peripheral-driving-circuit is constituted by a cMOSTFT, nMOSTFT, pMOSTFT or a combination thereof.
It is also an object to implement a large-size display panel having high image quality, high definition, narrow peripheral frame and high efficiency, while allowing the use of a large-sized glass substrate having a comparatively low distortion point, and while achieving a high yield and reduction in the production cost due to elimination of necessity for the use of expensive production facilities, as well as easy control of the threshold value which permits reduction in the electrical resistance to offer high-speed of operation and greater size of the display.
To these ends, according to one aspect of the present invention, there is provided a method of producing an electrooptical device having a first substrate, i.e., a driving substrate, carrying a display section provided with pixel electrodes, e.g., pixel electrodes arranged in the form of a matrix, and a peripheral-driving-circuit section provided on a periphery of the display section, a second substrate, i.e., a counter substrate, and an optical material such as a liquid crystal disposed between the first substrate and the second substrate, as well as a method for producing the driving substrate for such an electrooptical device; the method comprising the steps of: a gate-forming step for forming a gate portion including a gate electrode and a gate insulating film on one face of the first substrate; a step-forming step for forming a step difference on the one face of the first substrate; a layer-forming step for forming a polycrystalline or amorphous silicon layer having a predetermined thickness on the first substrate having the gate portion and the step difference and then forming a low-melting-point metal layer on or under the polycrystalline or amorphous silicon layer, or of forming a low-melting-point metal layer containing silicon on the first substrate having the step difference; a heating step for dissolving silicon of the polycrystalline or amorphous layer or of the low-melting-point metal layer into the low-melting-point metal layer by heating; a deposition step for depositing on the first substrate a single-crystal semiconductor layer by allowing the silicon of the polycrystalline or amorphous silicon layer or of the low-melting-point metal layer to grow by graphoepitaxy by a cooling treatment using as a seed the step difference on the substrate; a treating step-for effecting a predetermined treatment on the single-crystal semiconductor layer, thereby forming a channel region, a source region and a drain region; and a step for forming a first thin-film transistor MOSTFT) of dual-gate type having the gate portions on the above and below the channel region and constituting at least part of the peripheral-driving-circuit section. In accordance with the present invention, the thin-film transistor may be either a field effect transistor (FET) or a bipolar transistor, and the FET may be either a MOSFET or a junction type.
The present invention offers the following remarkable advantages (A) to (G), by virtue of the use of a single-crystal silicon layer as a dual-gate MOSTFT of a peripheral driving circuit as a driving substrate such as an active matrix substrate or as a dual-gate MOSTFT of a peripheral driving circuit of an electrooptical device such as an LCD of the type having a display-driver integrated structure, wherein the single-crystal silicon layer is graphoepitaxially grown from a polycrystalline or amorphous silicon layer or from a low-melting-point metal layer using the step difference formed on the substrate as a seed.
(A) It is possible to produce an electrooptical device such as a display thin-film semiconductor device incorporating a high-performance driver, by virtue of the use of a single-crystal silicon layer having a high electron mobility of 540 cm2/v.sec or higher that has been grown by graphoepitaxy conducted by using as a seed the bottom corner of a step of a predetermined size and shape formed on a substrate. Preferably, the step is formed to provide an indented section having such a cross-section that both side faces of the indented section are perpendicular to the bottom or slanted to form a basilar angle of not greater than 90xc2x0 with respect to the bottom face of the indented section.
(B) Since the single-crystal silicon thin-film has high electron and hole mobility, single-crystal silicon top-gate MOSTFTs can form a structure having a display section and a peripheral driving circuit section integrated with each other, wherein the display section has nMOSTFTs, pMOSTFTs or cMOSTFTs having high switching characteristics and, preferably, a lightly-doped drain (LDD) structure that reduces leak current through reducing the electric field intensity, while the peripheral driving circuit includes cMOSTFTs, nMOSTFTs, pMOSTFTs, or a combination thereof, having high driving characteristics, whereby production of a large display panel with high quality, high definition, a narrow frame, and high efficiency is facilitated. In contrast to pMOSTFT that can hardly provide high hole mobility, the single-crystal silicon thin-film has high hole mobility and, therefore, a peripheral driving circuit for driving electrons and holes independently or in combination can be implemented and integrated with display TFTs of nMOS, pMOS or cMOS LDD-type. In a compact or small panel, either of a pair of vertical peripheral driving circuits may be omitted.
(C) In particular, the use of dual-gate MOSTFTs in the peripheral driving circuit makes it possible to obtain cMOS, nMOS or pMOSTFTs having driving power 1.5 to 2.0 times as large that obtainable with the use of single-gate TFTs, thus achieving higher performance and greater driving power of the TFTs, offering advantages particularly when TFTs having large driving power are to be used in a local portion of the peripheral driving circuit. For instance, this feature not only allows omission of one of a pair of peripheral vertical driver circuits but also enables the present invention to be advantageously applied to organic ELs and FEDs. Furthermore, the dual-gate structure can easily be changed to a top-gate type structure or a bottom-gate type structure through a selection of one of the upper and lower gates and, in addition, ensures safe operation even in the event of a failure in one of the upper and lower gates because the other gate can safely be used.
(D) The polycrystalline or amorphous silicon layer can be formed by a plasma-enhanced or reduced-pressure CVD process at a substrate temperature of 100 to 400xc2x0 C., by using the above-mentioned step difference as a seed for the graphoepitaxy, and the low-melting-point metal layer can be formed by a vacuum evaporation process or a sputtering process and, in addition, the above-mentioned epitaxial growth of silicon can be achieved at a comparatively low heating temperature of, for example, 930xc2x0 C., so that the single-crystal silicon layer can be uniformly formed on the insulating substrate at a relatively low temperature of, for example, 400 to 450xc2x0 C.
(E) The method in accordance with the present invention does not include annealing at a middle temperature (approximately 600xc2x0 C.) for more than ten hours and excimer laser annealing that are required in solid-phase growth, thus offering a greater yield and reduction in the production cost due to elimination of the use of an expensive production facility.
(F) In the graphoepitaxy in the present invention, a single-crystal silicon thin-film having a variety of P-type impurity concentration and high mobility can be readily produced by adjusting the ratio of polycrystalline silicon or amorphous silicon to a low-melting-point metal, the heating temperature of the substrate, and the cooling rate, thus enabling easy control of the threshold voltage (Vth) of the device, which in turn enables the device to operate at a high speed due to reduced resistance.
(G) The low-melting-point metal layer containing polycrystalline or amorphous silicon when deposited may be doped with an adequate amount of Group III or V impurity, such as boron, phosphorus, antimony, arsenic, bismuth or aluminum, so that the type and the concentration of the impurity in the epitaxial single-crystal silicon, that is, the type (P-type or N-type) and the carrier concentration, are controllable.
In accordance with the present invention, it is preferred to form the aforementioned step difference in the insulating substrate or in a diffusion barrier such as a film of silicon nitride (referred to as SiN, hereinafter) or both in the insulating substrate and the diffusion barrier, so as to provide an indented section having such a cross-section that both side-faces of the indented section are perpendicular to the bottom or slanted to form a basilar angle of not greater than 90xc2x0 with respect to the bottom face of the indented section, and to use this step difference as a seed for the graphoepitaxy of the single-crystal silicon layer. Preferably, the step difference is formed along at least one side of a device region including the channel region, the source region and the drain region of the first thin-film transistor. When a passive device such as a resistor is formed of the aforementioned single-crystal silicon layer, the step difference is formed along at least one side of the device region in which the resistor is formed.
The first thin-film transistor such as MOSTFT may be formed inside, outside or both inside and outside the indented section defined by the step difference.
The step difference may be formed by a dry etching process, such as a reactive ion etching process, and the polycrystalline or amorphous silicon layer is formed preferably by a low-temperature deposition process at a substrate temperature of 100 to 400xc2x0 C., for example, a reduced-pressure CVD process, a catalytic CVD process, a plasma-enhanced CVD process, or a sputtering process so that the thickness becomes several xcexcm to 0.005 xcexcm, followed by the deposition of the low-melting-point metal layer by a vacuum evaporation process or a sputtering process so that the thickness becomes several tens to several hundreds times the thickness of the polycrystalline or amorphous silicon layer, and then the aforesaid heating treatment is executed.
In this case, the low-melting-point metal layer may be formed above or below the polycrystalline or amorphous silicon layer formed by the low-temperature deposition process. Alternatively, the aforementioned low-melting-point metal layer containing silicon is deposited and then subjected to the heating treatment.
The substrate may be an insulating substrate, for example, a glass substrate or a heat-resistant organic substrate, and the low-melting-point metal layer may be formed of at least one metal selected from the group consisting of indium, gallium, tin, bismuth, lead, zinc, antimony and aluminum.
When the low-melting-point metal layer is formed of indium, this layer is heated preferably at 850 to 1,100xc2x0 C. and more preferably 900 to 950xc2x0 C. in a hydrogen-based atmosphere (pure hydrogen, a nitrogen-hydrogen mixture, or an argon-hydrogen mixture) to form an indium-silicon melt and, when the low-melting-point metal layer is formed of indium-gallium, this layer is heated preferably at 300 to 1,100xc2x0 C. and more preferably 350 to 600xc2x0 C. in a hydrogen-based atmosphere to form an indium-gallium-silicon melt, whereas, when the low-melting-point metal layer is formed of gallium, this layer is heated preferably at 400 to 1,100xc2x0 C. and more preferably 420 to 600xc2x0 C. in a hydrogen-based atmosphere to form a gallium-silicon melt. The substrate may be uniformly heated using an electrical furnace or a lamp or, alternatively, a predetermined region of the substrate may be locally heated using laser or electron beams.
With reference to. FIG. 11, the melting point of the silicon-containing low-melting-point metal decreases as the content of the low-melting-point metal increases. The indium melt layer containing silicon, e.g., by 1 percent by weight, is formed at a substrate temperature of 850 to 1,100xc2x0 C. when indium is used, because such a substrate temperature facilitates use of glass having low heat resistance, such as crystallized glass, in addition to quartz glass, as a substrate. A gallium melt layer containing 1 percent by weight of silicon may be formed at a temperature of 400 to 1,100xc2x0 C. on any glass substrate.
When indium-gallium-silicon or gallium-silicon is used, a glass substrate having a low distortion point or a heat-resistant organic substrate can be used, so that a semiconductive crystalline layer can be formed on a large glass substrate having an area of, for example, 1 m2, which is inexpensive and can readily be prepared in the form of a rolled-long glass sheet. A single-crystal silicon thin-film can be formed continuously or discontinuously on the long glass or organic substrate by the described process employing graphoepitaxy.
Since the components of the glass having the low distortion point rapidly diffuse into the upper layer, a thin diffusion-barrier layer composed of, for example, silicon nitride having a thickness of 50 to 200 nm is preferably formed to suppress such diffusion. Thus, the polycrystalline or amorphous silicon layer or a silicon-containing low-melting-point metal layer is formed on the diffusion-barrier layer.
The silicon-containing low-melting-point metal layer is slowly cooled so that the single-crystal silicon layer is deposited by graphoepitaxy using the step difference as a seed, and then the low-melting-point metal layer is removed by, for example, hydrochloric acid, followed by a predetermined treatment to form an active device and a passive device.
After the low-melting-point metal layer such as of indium deposited on the single-crystal silicon layer after the cooling is dissolved and removed by, for example, hydrochloric acid, only a trace amount (approximately 1016 atoms/cc) of indium remains in the silicon layer, so that the single-crystal silicon layer becomes a p-type thin-film semiconductor. This layer is advantageous for production of a nMOSTFT. An n-type impurity such as phosphorus may be ion-implanted into the entire surface or selective regions of the single-crystal silicon layer to form an n-type single-crystal silicon thin-film, whereby a pMOSTFT can also be obtained. A cMOSTFT can also be formed. The polycrystalline or amorphous silicon layer or the silicon-containing low-melting-point metal layer may be doped with a Group III or V impurity having a large solubility, such as boron, phosphorus, antimony, arsenic, or bismuth, in a proper amount, during the deposition of this layer so as to control the type and/or the concentration of the impurity in the epitaxially grown silicon layer, that is, to control the doping type (n- or p-) and/or the concentration of the carrier.
Accordingly, the single-crystal silicon layer grown by graphoepitaxy on the substrate is used as a-channel region, a source region and a drain region of a dual-gate-MOSTFT which constitutes at least a part of the peripheral driving circuit, the type and the concentration of each region being controllable, as described above.
Thin-film transistors in the peripheral-driving-circuit section and the display section may constitute n-channel, p-channel or complementary insulating-gate field-effect transistors: for example, a thin-film transistor may comprise a combination of a complementary type and an n-channel type, a complementary type and a p-channel type, or a complementary type, an n-channel type and a p-channel type. Preferably at least a part of the thin-film transistors in the peripheral-driving-circuit section and/or the display section has a lightly-doped drain (LDD) structure of a single type having a LDD section between the gate and the source or drain, or of a double type having LDD sections between the gate and source and between the gate and the drain, respectively.
Preferably, the MOSTFT constitutes an LDD-type TFT of an nMOS, a pMOS or a cMOS in the display section, and a cMOSTFT, an nMOSTFT, a pMOSTFT, or a mixture thereof, in the peripheral-driving-circuit section.
The MOSTFT is preferably formed on at least one of the interior and the exterior of the indented section of the substrate.
In such a case, the step difference is formed on one surface of the first substrate, and a single-crystal, polycrystalline or amorphous silicon layer is formed on the surface having the step difference. The single-crystal, polycrystalline or amorphous silicon layer is used as a channel region, a source region and a drain region of a second thin-film transistor, and at least one gate section is provided above and/or below the channel region. That is, the second thin-film transistor may be a top-gate, bottom-gate or dual-gate thin-film transistor.
In this case also, the step difference forms an indented section having a cross-section in which a side face is perpendicular to or slanted to the bottom face so as to have a basilar angle of preferably 90xc2x0 or less, and the step difference functions as a seed for graphoepitaxy of the single-crystal silicon layer.
The second thin-film transistor may be formed in a region including the indented section defined by the step difference formed in the first substrate and/or the film formed on the first substrate, and the graphoepitaxial single-crystal silicon layer may be used to form the source, drain and channel regions of the second thin-film transistor, as in the case of the first thin-film transistor.
In this second thin-film transistor also, the type and the concentration of the Group III or V impurity in the single-crystal, polycrystalline or amorphous silicon layer may be controlled as described above, and a step difference may be formed along at least one side of a device region including the channel region, the source region and the drain region of the second thin-film transistor. A gate electrode below the single-crystal, polycrystalline or amorphous silicon layer is preferably trapezoidal at the side end section. A diffusion-barrier layer may be provided between the-first substrate and the single-crystal, polycrystalline or amorphous silicon layer.
The source or drain electrodes of the first and/or second thin-film transistors are preferably formed on a region including the step difference.
The first thin-film transistor may comprise at least the dual-gate type among a top-gate type having a gate section above the channel region, a bottom-gate type having a gate section below the channel region, and a dual-gate type having one gate section above and one below the channel region, and the switching device may comprise one of a top-gate second thin-film transistor, a bottom-gate second thin-film transistor and a dual-gate second thin-film transistor.
In this case, the gate section formed below the channel region in the bottom-gate or dual-gate second thin-film transistor preferably comprises a heat resistant material, and an upper-gate electrode of the second thin-film transistor and a gate electrode of the first thin-film transistor may comprise a common material.
The peripheral-driving-circuit section may comprise, in addition to the above-mentioned first thin-film transistor, at least one of a top-gate, bottom-gate or dual-gate thin-film transistor having a channel region of a polycrystalline or amorphous silicon layer and a gate region formed above or below the channel region, and may further comprise a diode, a resistor, a capacitor and an inductor, each comprising a single-crystal, polycrystalline or amorphous silicon layer.
Thin-film transistors in the peripheral driving circuit and/or the display section have a single-gate or a multi-gate configuration.
Preferably, when the n- or p-channel thin-film transistor in the peripheral-driving-circuit section and/or the display section is a dual-gate type, the upper or a lower-gate electrode is electrically opened or a given negative voltage for the n-channel type or a given positive voltage for the p-channel type is applied so that the dual-gate type thin-film transistors operate as bottom- or top-gate type thin-film transistors.
The thin-film transistor in the peripheral-driving-circuit section may be the first thin-film transistor of an n-channel, p-channel or complementary type, and the thin-film transistor in the display section may be an n-channel, p-channel or complementary type when the channel region is a single-crystal silicon layer, a polycrystalline silicon layer, or an amorphous silicon layer.
After the single-crystal silicon layer is deposited, an upper-gate section including a gate insulating film and a gate electrode may be formed on the single-crystal silicon layer, and the single-crystal silicon layer may be doped with a Group III or V impurity through the upper-gate section to form the channel region, the source region and the drain region.
When the second thin-film transistor is a bottom-gate type or a dual-gate type, a lower-gate electrode composed of a heat resistant material is provided below the channel region, and a gate insulating film is formed on the gate electrode to form a lower-gate section, and the second thin-film transistor is formed by the same steps including the step for forming the step difference as those in the first thin-film transistor. In such a case the upper-gate electrode of the second thin-film transistor and the gate electrode of the first thin-film transistor may be composed of a common material.
The single-crystal silicon layer formed on the lower-gate section may be doped with a Group III or v impurity to form a source region and a drain region, followed by an activation treatment.
The source and drain regions of the second thin-film transistor may be formed by ion-implantation of the above impurity on the single-crystal silicon layer through a resist mask and may be subjected to activation treatment, and the gate electrode of the first thin-film transistor, and the upper-gate electrode of the second thin-film transistor, if necessary, may be-formed after the formation of the gate insulating film.
When the thin-film transistor is a top-gate type, the source and drain regions of the first and second thin-film transistors may be formed on the single-crystal silicon layer by ion implantation of the impurity through a resist mask and may be subjected to activation treatment, and then the gate sections including the gate insulating films and the gate electrodes of the first and second thin-film transistors may be formed.
Alternatively, when the thin-film transistor is of the top-gate type, the gate sections including the gate insulating films and the gate electrodes of the first and second thin-film transistors may be formed, and then the source and drain regions of the first and second thin-film transistors may be formed on the single-crystal silicon layer by ion implantation of the impurity through a resist mask, followed by an activation treatment.
The resist mask used when the LDD structure was formed may be left unremoved and the ion implantation for forming the source and drain regions may be performed through a resist mask which covers the remaining mask.
The substrate may be optically opaque or transparent, and may be provided with pixel electrodes for a reflective or transmissive display.
The display section may have a lamination configuration of the pixel electrodes and a color filter layer which may be formed on a display array, whereby the aperture ratio and the luminance are improved and costs can be decreased due to omission of a color filter substrate and improved productivity.
When the pixel electrodes are reflective electrodes, unevenness is preferably imparted to a resin film so that the resin film has optimized reflective characteristics and viewing-angle characteristics, and then the pixel electrodes are formed, whereas, when the pixel electrodes are transparent electrodes, the surface is preferably planarized by a transparent planarization film and then the pixel electrodes are formed on the planarized plane.
The display section is illuminated or dimmed by being driven by the above-described MOSTFT: for instance, the display section may comprise, for example, a liquid crystal display (LCD), an electroluminescent (EL) display, a field emission display (FED), a light-emitting polymer display (LEPD), or a light-emitting diode (LED) display. In this case, the arrangement may be such that a plurality of pixel electrodes are arranged in a matrix in the display section and a switching device is connected to each pixel electrode.
In accordance with a second aspect of the present invention, there is provided a method of producing an electrooptical device having a first substrate, i.e., a driving substrate, carrying a display section provided with pixel electrodes, e.g., pixel electrodes arranged in the form of a matrix, and a peripheral-driving-circuit section proivded on a periphery of the display section, a second substrate, i.e., a counter substrate, and an optical material such as a liquid crystald disposed between the first substrate and the second substrate, as well as a method for producing the driving substrate for such an electrooptical device, the method comprising the steps of:
a gete-forming step for forming a gate portion including a gaqte electrode andd a gate insulating filim on one face of the first substrate;
a step-forming step for forming a step difference on the one face of the first substrate;
a layer-forming step for forming a melt layer of a low-melting-point metal containing silicon on the first substrate having the gate portion and the step difference;
a deposition step for deposititing a single-crystal silicon layer by allowing the silicon of the melt layer to grow by graphoepitaxy by a cooling treatment using as a seed the step difference on the substrate;
a step for effecting a predetermined treatment on the single-crystal silicon layer, thereby forming a channel region, a source region and a drain region; and
a step for forming a first thin-film transistor (MOSTFT) of dual gate type having the gate portions on the above and below the channel region and constituting at least part of the peripheral-driving-sircuit section. In accordance with the second aspect of the present invention, the thin-film transistor may be either a field effect transistor (FET) or a bipolar transistor, and the FET may be either a MOSTFT or a junction type.
The second aspect of the present invention offers the following remarkable advantages (A) to (G), by virture of the use of a single-crystal silicon layer as a dual-gate MOSTFT of a peripheral driving circuit as a driving substrate such as an active matrix substrate or as a dual-gate MOSTFT of a peripheral driving circuit of an electrooptical device such as an LCD of the type having a display-driver integrated structure, wherein the single-crystal silicon layer is graphoepitaxially grown from a melt of a low-melting-point metal containing silicon using the step difference formed on the substrate as a seed.
(A) It is possible to produce an electrooptical device such as a display thin-film semiconductor device incorporating a high-performance driver, by virtue of the use of a single-crystal silicon layer having a high electron mobility of 540 cm2/v.sec or higher that has been grown by graphoepitaxy conducted by using as a-seed the bottom corner of a step of a predetermined size and shape formed on a substrate. Preferably, the step is formed to provide an indented section having such a cross-section that both side faces of the indented section are perpendicular to the bottom or slanted to form a basilar angle of not greater than 90xc2x0 with respect to the bottom face of the indented section.
(B) Since the single-crystal silicon thin-film has high electron and hole mobility, single-crystal silicon top-gate MOSTFTs can form a structure having a display section and a peripheral driving circuit section integrated with each other, wherein the display section has nMOSTFTs, pMOSTFTs or cMOSTFTs having high switching characteristics and, preferably, a lightly-doped drain (LDD) structure that reduces leak current through reducing the electric field intensity, while the peripheral driving circuit includes cMOSTFTs, nMOSTFTs, pMOSTFTs, or a combination thereof, having high driving characteristics, whereby production of a large display panel with high quality, high definition, a narrow frame, and high efficiency is facilitated. In contrast to pMOSTFT that can hardly provide high hole mobility, the single-crystal silicon thin-film has high hole mobility and, therefore, a peripheral driving circuit for driving electrons and holes independently or in combination can be implemented and integrated with display TFTs of nMOS, pMOS or cMOS LDD-type. In compact to medium-sized panels, one of a pair of vertical peripheral driving circuits may be omitted.
(C) In particular, the use of dual-gate MOSTFTs in the peripheral driving circuit makes it possible to obtain cMOS, nMOS or pMOSTFTs having driving power 1.5 to 2.0 times as large that obtainable with the use of single-gate TFTs, thus achieving higher performance and greater driving power of the TFTs, offering advantages particularly when TFTs having large driving power are to be used in a local portion of the peripheral driving circuit. For instance, this feature not only allows omission of one of a pair of peripheral vertical driver circuits but also enables the present invention to be advantageously applied to organic ELs and FEDs. Furthermore, the dual-gate structure can easily be changed to a top-gate type structure or a bottom-gate type structure through a selection of one of the upper and lower gates and, in addition, ensures safe operation even in the event of a failure in one of the upper and lower gates because the other gate can safely be used.
(D) The melt of the low-melting-point metal is prepared at a temperature of, for example, 350xc2x0 C., by using the above-mentioned step difference as a seed for the graphoepitaxy, and the melt can be fromed on the substrate which is heated at a temperature slightly higher than the temperature of the melt by an application process, so that the single-crystal silicon film can be uniformly formed at a relatively low temperature of, for example, 300 to 400xc2x0 C.
(E) The method in accordance with the present invention does not include annealing at a middle temperature (approximately 600xc2x0 C.) for more than ten hours and excimer laser annealing that are required in solid-phase growth, thus offering a greater yield and reduction in the production cost due to elimination of the use of an expensive production facility.
(F) In the graphoepitaxy, a single-crystal silicon layer having a variety of P-type impurity concentration and high mobility can be readily produced by adjusting the composition ratio of the melt, the temperature of the melt, the heating temperature of the substrate, and the cooling rate, thus enabling easy control of the threshold voltage (Vth) of the device, which in turn enables the device to operate at a high speed due to reduced resistance.
(G) The melt layer of the low-melting-point metal containing silicon may be doped with an adequate amount of Group III or V impurity, such as boron, phosphorus, antimony, arsenic, bismuth or aluminum, so that the type and the concentration of the impurity in the epitaxial single-crystal silicon layer, that is, the type (P-type or N-type) and the carrier concentration, are controllable.
The step difference may be formed by a dry etching process, such as a reactive ion etching process, and the melt of the low-melting-point metal containing 2.0 to 0.005 percent by weight of, for example, 1 percent by weight of silicon may be applied to the heated insulating substrate and maintained for a predetermined period, for example, several to several tens of minutes, and then a cooling treatment may be performed. This can provide a single-crystal silicon layer having a thickness of several to 0.005 xcexcm, for example, 1 xcexcm.
The substrate may be an insulating substrate, for example, a glass substrate or a heat-resistant organic substrate, and the low-melting-point metal may comprise at least one metal selected from the group consisting of indium, gallium, tin, bismuth, lead, zinc, antimony, and aluminum.
When the low-melting-point metal comprises indium, the melt may be applied to the insulating substrate heated preferably at a temperature of 850 to 1,100xc2x0 C. and more preferably 900 to 950xc2x0 C. and when the low-meting-point metal comprises indium-gallium, the melt may be applied to the insulating substrate heated preferably at a temperature of 300 to 1,100xc2x0 C. and more preferably 350 to 600xc2x0 C., whereas when the low-melting-point layer comprises gallium, the melt may be applied to the insulating substrate heated preferably at a temperature of 400 to 1,100xc2x0 C. and more preferably 420 to 600xc2x0 C. The substrate may be uniformly heated using an electrical furnace or a lamp or, alternatively, a predetermined region of the substrate may be locally heated using laser or electron beams.
With reference to FIG. 11, the melting point of the silicon-containing low-melting-point metal decreases as the content of the low-melting-point metal increases. The indium melt layer containing silicon, e.g., by 1 percent by weight, is formed at a substrate temperature of 850 to 1,100xc2x0 C. when indium is used, because such a substrate temperature facilitates use of glass having low heat resistance, such as crystallized glass, in addition to quartz glass, as a substrate. A gallium melt layer containing 1 percent by weight of silicon may be formed at a temperature of 400 to 1,100xc2x0 C. on any glass substrate.
When indium-gallium-silicon or gallium-silicon is used, a glass substrate having a low distortion point or a heat-resistant organic substrate can be used, so that a semiconductive crystalline layer can be formed on a large glass substrate having an area of, for example, 1 m2, which is inexpensive and can readily be prepared in the form of a rolled long glass sheet. A single-crystal silicon thin-film can be formed continuously or discontinuously on the long glass or organic substrate by the described process employing graphoepitaxy.
While the substrate is cooled after being maintained for a fixed period in the aforementioned application process, a dipping process in which the glass substrate is dipped with the melt and maintained for a fixed period and then, the substrate is gradually pulled up, or a floating process in which the substrate is cooled while being moved in the melt or while the surface of the substrate is being moved at an adequate speed may be employed. The thickness of an eptaxially grown layer, and the carrier impurity concentration can be controlled by the composition ratio and the temperature of the melt and the pulling-up speed. The substrate can be continuously or intermittently processed by the application process, dipping process and floating process, so that mass productivity is increased.
Since the components of the glass having the low distortion point rapidly diffuse into the upper layer, a thin diffusion-barrier layer composed of, for example, silicon nitride having a thickness of 50 to 200 nm is preferably formed to suppress such diffusion. Thus, the polycrystalline or amorphous silicon layer or a silicon-containing low-melting-point metal layer is formed on the diffusion-barrier layer.
The silicon-containing low-melting-point metal layer is slowly cooled so that the single-crystal silicon layer is deposited by graphoepitaxy using the step difference as a seed, and then the low-melting-point metal layer is removed by, for example, hydrochloric acid, followed by a predetermined treatment to form an active device and a passive device.
After the low-melting-point metal layer such as of indium deposited on the single-crystal silicon layer after the cooling is dissolved and removed by, for example, hydrochloric acid, only a trace amount (approximately 1016 atoms/cc) of indium remains in the silicon layer, so that the single-crystal silicon layer becomes a p-type thin-film semiconductor. This layer is advantageous for production of a nMOSTFT. An n-type impurity such as phosphorus may be ion-implanted into the entire surface or selective regions of the single-crystal silicon layer to form an n-type single-crystal silicon thin-film, whereby a pMOSTFT can also be obtained. A cMOSTFT can also be formed. The polycrystalline or amorphous silicon layer or the silicon-containing low-melting-point metal layer may be doped with a Group III or V impurity having a large solubility, such as boron, phosphorus, antimony, arsenic, or bismuth, in a proper amount, during the deposition of this layer so as to control the type and/or the concentration of the impurity in the epitaxially grown silicon layer, that is, to control the doping type (n- or p-) and/or the concentration of the carrier.
Accordingly, the single-crystal silicon layer grown by graphoepitaxy on the substrate is used as a channel region, a source region and a drain region of a top-gate MOSTFT which constitutes at least a part of the peripheral driving circuit, the type and the concentration of each region being controllable, as described above.
Thin-film transistors in the peripheral-driving-circuit section and the display section may constitute n-channel, p-channel or complementary insulating-gate field-effect transistors: for example, a thin-film transistor may comprise a combination of a complementary type and an n-channel type, a complementary type and a p-channel type, or a complementary type, an n-channel type and a p-channel type. Preferably at least a part of the thin-film transistors in the peripheral-driving-circuit section and/or the display section has a lightly-doped drain (LDD) structure of a single type having a LDD section between the gate and the source or drain, or of a double type having LDD sections between the gate and source and between the gate and the drain, respectively.
Preferably, the MOSTFT constitutes an LDD-type TFT of an nMOS, a pMOS or a CMOS in the display section, and a cMOSTFT, an nMOSTFT, a pMOSTFT, or a mixture thereof, in the peripheral-driving-circuit section.
The MOSTFT is preferably formed on at least one of the interior and the exterior of the indented section of the substrate.
In such a case, the step difference is formed on one surface of the first substrate, and a single-crystal, polycrystalline or amorphous silicon layer is formed on the surface having the step difference. The single-crystal, polycrystalline or amorphous silicon layer is used as a channel region, a source region and a drain region of a second thin-film transistor, and at least one gate section is provided above and/or below the channel region. That is, the second thin-film transistor may be a top-gate, bottom-gate or dual-gate thin-film transistor.
In this case also, the step difference forms an indented section having a cross-section in which a side face is perpendicular to or slanted to the bottom face so as to have a basilar angle of preferably 90xc2x0 or less, and the step difference functions as a seed for graphoepitaxy of the single-crystal silicon layer.
The second thin-film transistor may be formed in a region including the indented section defined by the step difference formed in the first substrate and/or the film formed on the first substrate, and the graphoepitaxial single-crystal silicon layer may be used to form the source, drain and channel regions of the second thin-film transistor, as in the case of the first thin-film transistor.
In this second thin-film transistor also, the type and the concentration of the Group III or V impurity in the single-crystal, polycrystalline or amorphous silicon layer may be controlled as described above, and a step difference may be formed along at least one side of a device region including the channel region, the source region and the drain region of the second thin-film transistor. A gate electrode below the single-crystal, polycrystalline or amorphous silicon layer is preferably trapezoidal at the side end section. A diffusion-barrier layer may be provided between the first substrate and the single-crystal, polycrystalline or amorphous silicon layer.
The source or drain electrodes of the first and/or second thin-film transistors are preferably formed on a region including the step difference.
The first thin-film transistor may comprise at least the top-gate type among a top-gate type having a gate section above the channel region, a bottom-gate type having a gate section below the channel region, and a dual-gate type having one gate section above and one below the channel region, and the switching device may comprise one of a top-gate second thin-film transistor, a bottom-gate second thin-film transistor and a dual-gate second thin-film transistor.
In this case, the gate section formed below the channel region in the bottom-gate or dual-gate second thin-film transistor preferably comprises a heat resistant material, and an upper-gate electrode of the second thin-film transistor and a gate electrode of the first thin-film transistor may comprise a common material.
The peripheral-driving-circuit section may comprise, in addition to the above-mentioned first thin-film transistor, at least one of a top-gate, bottom-gate or dual-gate thin-film transistor having a channel region of a polycrystalline or amorphous silicon layer and a gate region formed above or below the channel region, and may further comprise a diode, a resistor, a capacitor and an inductor, each comprising a single-crystal, polycrystalline or amorphous silicon layer.
Thin-film transistors in the peripheral driving circuit and/or the display section have a single-gate or a multi-gate configuration.
Preferably, when the n- or p-channel thin-film transistor in the peripheral-driving-circuit section and/or the display section is a dual-gate type, the upper or a lower-gate electrode is electrically opened or a given negative voltage for the n-channel type or a given positive voltage for the p-channel type is applied so that the dual-gate type thin-film transistors operate as bottom- or top-gate type thin-film transistors.
The thin-film transistor in the peripheral-driving-circuit section may be the first thin-film transistor of an n-channel, p-channel or complementary type, and the thin-film transistor in the display section may be an n-channel, p-channel or complementary type when the channel region is a single-crystal silicon layer, a polycrystalline silicon layer, or an amorphous silicon layer.
After the single-crystal silicon layer is deposited, an upper-gate section including a gate insulating film and a gate electrode may be formed on the single-crystal silicon layer, and the single-crystal silicon layer may be doped with a Group III or V impurity through the upper-gate section to form the channel region, the source region and the drain region.
When the second thin-film transistor is a bottom-gate type or a dual-gate type, a lower-gate electrode composed of a heat resistant material is provided below the channel region, and a gate insulating film is formed on the gate electrode to form a lower-gate section, and the second thin-film transistor is formed by the same steps including the step for forming the step difference as those in the first thin-film transistor. In such a case the upper-gate electrode of the second thin-film transistor and the gate electrode of the first thin-film transistor may be composed of a common material.
The single-crystal silicon layer formed on the lower-gate section may be doped with a Group III or V impurity to form a source region and a drain region, followed by an activation treatment.
The source and drain regions of the second thin-film transistor may be formed by ion-implantation of the above impurity on the single-crystal silicon layer through a resist mask and may be subjected to activation treatment, and the gate electrode of the first thin-film transistor, and the upper-gate electrode of the second thin-film transistor, if necessary, may be formed after the formation of the gate-insulating film.
When the thin-film transistor is a top-gate type, the source and drain regions of the first and second thin-film transistors may be formed on the single-crystal silicon layer by ion implantation of the impurity through a resist mask and may be subjected to activation treatment, and then the gate sections including the gate insulating films and the gate electrodes of the first and second thin-film transistors may be formed.
Alternatively, when the thin-film transistor is of the top-gate type, the gate sections including the gate insulating films and the gate electrodes of the first and second thin-film transistors may be formed, and then the source and drain regions of the first and second thin-film transistors may be formed on the single-crystal silicon layer by ion implantation of the impurity through a resist mask, followed by an activation treatment.
The resist mask used when the LDD structure was formed may be left unremoved and the ion implantation for forming the source and drain regions may be performed through a resist mask which covers the remaining mask.
The substrate may be optically opaque or transparent, and may be provided with pixel electrodes for a reflective or transmissive display.
The display section may have a lamination configuration of the pixel electrodes and a color filter layer which may be formed on a display array, whereby the aperture ratio and the luminance are improved and costs can be decreased due to omission of a color filter substrate and improved productivity.
When the pixel electrodes are reflective electrodes, unevenness is preferably imparted to a resin film so that the resin film has optimized reflective characteristics and viewing-angle characteristics, and then the pixel electrodes are formed, whereas, when the pixel electrodes are transparent electrodes, the surface is preferably planarized by a transparent planarization film and then the pixel electrodes are formed on the planarized plane.
The display section is illuminated or dimmed by being driven by the above-described MOSTFT: for instance, the display section may comprise, for example, a liquid crystal display (LCD), an electroluminescent (EL) display, a field emission display (FED), a light-emitting polymer display (LEPD), or a light-emitting diode (LED) display. In this case, the arrangement may be such that a plurality of pixel electrodes are arranged in a matrix in the display section and a switching device is connected to each pixel electrode.
In accordance with a third aspect of the present invention, there is provided a method of producing an electrooptical device, as well as a method of producing a driving substrate for such an electrooptical device, having a first substrate carrying a display section provided with pixel electrodes and a peripheral-driving-circuit section provided on a periphery of the display section, a second substrate, and an optical material disposed between the first substrate and the second substrate; the method comprising the steps of: a gate-forming step for forming a gate portion including a gate electrode and a gate insulating film on one face of the first substrate; a step-forming step for forming a step difference on the one face of the first substrate; a layer-forming step for forming a polycrystalline or amorphous silicon layer having a predetermined thickness on the first substrate having the gate portion and the step difference and then forming a low-melting-point metal layer on or under the polycrystalline or amorphous silicon layer, or of forming a low-melting-point metal layer containing silicon on the first substrate having the step difference; a heating step for dissolving silicon of the polycrystalline or amorphous layer or of the low-melting-point metal layer into the low-melting-point metal layer by heating; a deposition step for depositing on the first substrate a single-crystal silicon layer by allowing the silicon of the polycrystalline or amorphous silicon layer or of the low-melting-point metal layer to grow by graphoepitaxy by a cooling treatment using as a seed the step difference on the substrate; a step for effecting a predetermined treatment on the single-crystal silicon layer, thereby forming a channel region, a source region and a drain region; and a step for forming a first thin-film transistor of bottom-gate type having the gate portions on the below the channel region and constituting at least part of the peripheral-driving-circuit section. The above-mentioned thin-film transistor may be a field effect transistor (FET) or a bipolar transistor. The FET, when used, may be a MOS-type FET or a junction-type FET.
In accordance with this aspect of the present invention, the same advantages as those offered by the first aspect of the invention are obtainable, except for the advantage derived in the first aspect from the use of the dual-gate structure of the thin-film transistor.
In accordance with the present invention, it is preferred to form the aforementioned step difference in the insulating substrate or in a diffusion barrier such as a film of silicon nitride (referred to as SiN, hereinafter) or both in the insulating substrate and the diffusion barrier, so as to provide an indented section having such a cross-section that both side faces of the indented section are perpendicular to the bottom or slanted to form a basilar angle of not greater than 90xc2x0 with respect to the bottom face of the indented section, and to use this step difference as a seed for the graphoepitaxy of the single-crystal silicon layer. Preferably, the step difference is formed along at least one side of a device region including the channel region, the source region and the drain region of the first thin-film transistor. When a passive device such as a resistor is formed of the aforementioned single-crystal silicon layer, the step difference is formed along at least one side of the device region in which the resistor is formed.
The first thin-film transistor such as MOSTFT may be formed inside, outside or both inside and outside the indented section defined by the step difference.
The step difference may be formed by a dry etching process, such as a reactive ion etching process, and the polycrystalline or amorphous silicon layer is formed preferably by a low-temperature deposition process at a substrate temperature of 100 to 400xc2x0 C., for example, a reduced-pressure CVD process, a catalytic CVD process, a plasma-enhanced CVD process, or a sputtering process so that the thickness becomes several xcexcm to 0.005 xcexcm, followed by the deposition of the low-melting-point metal layer by a vacuum evaporation process or a sputtering process so that the thickness becomes several tens to several hundreds times the thickness of the polycrystalline or amorphous silicon layer, and then the heating treatment is executed.
In this case, the low-melting-point metal layer may be formed above or below the polycrystalline or amorphous silicon layer formed by the low-temperature deposition process. Alternatively, the aforementioned low-melting-point metal layer containing silicon is deposited and then subjected to the heating treatment.
The substrate may be an insulating substrate, for example, a glass substrate or a heat-resistant organic substrate, and the low-melting-point metal layer may be formed of at least one metal selected from the group consisting of indium, gallium, tin, bismuth, lead, zinc, antimony and aluminum.
When the low-melting-point metal layer is formed of indium, this layer is heated preferably at 850 to 1,100xc2x0 C. and more preferably 900 to 950xc2x0 C. in a hydrogen-based atmosphere (pure hydrogen, a nitrogen-hydrogen mixture, or an argon-hydrogen mixture) to form an indium-silicon melt and, when the low-melting-point metal layer is formed of indium-gallium, this layer is heated preferably at 300 to 1,100xc2x0 C. and more preferably 350 to 600xc2x0 C. in a hydrogen-based atmosphere to form an indium-gallium-silicon melt, whereas, when the low-melting-point metal layer is formed of gallium, this layer is heated preferably at 400 to 1,100xc2x0 C. and more preferably 420 to 600xc2x0 C. in a hydrogen-based atmosphere to form a gallium-silicon melt. The substrate may be uniformly heated using an electrical furnace or a lamp or, alternatively, a predetermined region of the substrate may be locally heated using laser or electron beams.
With reference to FIG. 11, the melting point of the silicon-containing low-melting-point metal decreases as the content of the low-melting-point metal increases. The indium melt layer containing silicon, e.g., by 1 percent by weight, is formed at a substrate temperature of 850 to 1,100xc2x0 C. when indium is used, because such a substrate temperature facilitates use of glass having low heat resistance, such as crystallized glass, in addition to quartz glass, as a substrate. A gallium melt layer containing 1 percent by weight of silicon may be formed at a temperature of 400 to 1,100xc2x0 C. on any glass substrate.
When indium-gallium-silicon or gallium-silicon is used, a glass substrate having a low distortion point or a heat-resistant organic substrate can be used, so that a single-crystal silicon layer can be formed on a large glass substrate having an area of, for example, 1 m2, which is inexpensive and can readily be prepared in the form of a rolled long glass sheet.
Since the components of the glass having the low distortion point rapidly diffuse into the upper layer, a thin diffusion-barrier layer composed of, for example, silicon nitride having a thickness of 50 to 200 nm is preferably formed to suppress such diffusion. Thus, the polycrystalline or amorphous silicon layer or a silicon-containing low-melting-point metal layer is formed on the diffusion-barrier layer.
The silicon-containing low-melting-point metal layer is slowly cooled so that the single-crystal silicon layer is deposited by graphoepitaxy using the step difference as a seed, and then the low-melting-point metal layer is removed by, for example, hydrochloric acid, followed by a predetermined treatment to form an active device and a passive device.
After the low-melting-point metal layer such as of indium deposited on the single-crystal silicon layer after the cooling is dissolved and removed by, for example, hydrochloric acid, only a trace amount (approximately 1016 atoms/cc) of indium remains in the silicon layer, so that the single-crystal silicon layer becomes a p-type thin-film semiconductor. This layer is advantageous for production of a nMOSTFT. An n-type impurity such as phosphorus may be ion-implanted into the entire surface or selective regions of the single-crystal silicon layer to form an n-type single-crystal silicon thin-film, whereby a pMOSTFT can also be obtained. A cMOSTFT can also be formed. A cMOSTFT can also be formed. The polycrystalline or amorphous silicon layer or the silicon-containing low-melting-point metal layer may be doped with a Group III or V impurity having a large solubility, such as boron, phosphorus, antimony, arsenic, or bismuth, in a proper amount, during the deposition of this layer so as to control the type and/or the concentration of the impurity in the epitaxially grown silicon layer, that is, to control the doping type (n- or p- and/or the concentration of the carrier.
Accordingly, the single-crystal silicon layer grown by graphoepitaxy on the substrate is used as a channel region, a source region and a drain region of a dual-gate MOSTFT which constitutes at least a part of the peripheral driving circuit, the type and the concentration of each region being controllable, as described above.
Thin-film transistors in the peripheral-driving-circuit section and the display section may constitute n-channel, p-channel or complementary insulating-gate field-effect transistors: for example, a thin-film transistor may comprise a combination of a complementary type and an n-channel type, a complementary type and a p-channel type, or a complementary type, an n-channel type and a p-channel type. Preferably at least a part of the thin-film transistors in the peripheral-driving-circuit section and/or the display section has a lightly-doped drain (LDD) structure of a single type having a LDD section between the gate and the source or drain, or of a double type having LDD sections between the gate and source and between the gate and the drain, respectively.
Preferably, the MOSTFT constitutes an LDD-type TFT of an nMOS, a pMOS or a cMOS in the display section, and a cMOSTFT, an nMOSTFT, a pMOSTFT, or a mixture thereof, in the peripheral-driving-circuit section.
The MOSTFT is preferably formed on at least one of the interior and the exterior of the indented section of the substrate.
In, such a case, the step difference is formed on one surface of the first substrate, and a single-crystal, polycrystalline or amorphous silicon layer is formed on the surface having the step difference. The single-crystal, polycrystalline or amorphous silicon layer is used as a channel region, a source region and a drain region of a second thin-film transistor, and at least one gate section is-provided above and/or below the channel region. That is, the second thin-film transistor may be a top-gate, bottom-gate or dual-gate thin-film transistor.
In this case also, the step difference forms an indented section having a cross-section in which a side face is perpendicular to or slanted to the bottom face so as to have a basilar angle of preferably 90xc2x0 or less, and the step difference functions as a seed for graphoepitaxy of the single-crystal silicon layer.
The second thin-film transistor may be formed in a region including the indented section defined by the step difference formed in the first substrate and/or the film formed on the first substrate, and the graphoepitaxial single-crystal silicon layer may be used to form the source, drain and channel regions of the second thin-film transistor, as in the case of the first thin-film transistor.
In this second thin-film transistor also, the type and the concentration of the Group III or V impurity in the single-crystal, polycrystalline or amorphous silicon layer may be controlled as described above, and a step difference may be formed along at least one side of a device region including the channel region, the source region and the drain region of the second thin-film transistor. A gate electrode below the single-crystal, polycrystalline or amorphous silicon layer is preferably trapezoidal at the side end section. A diffusion-barrier layer may be provided between the first substrate and the single-crystal, polycrystalline or amorphous silicon layer.
The source or drain electrodes of the first and/or second thin-film transistors are preferably formed on a region including the step difference.
The first thin-film transistor may comprise at least the bottom-gate type among a top-gate type having a gate section above the channel region, a bottom-gate type having a gate section below the channel region, and a dual-gate type having one gate section above and one below the channel region, and the switching device may comprise one of a top-gate second thin-film transistor, a bottom-gate second thin-film transistor and a dual-gate second thin-film transistor.
In this case, the gate section formed below the channel region in the bottom-gate or dual-gate second thin-film transistor preferably comprises a heat resistant material, and an upper-gate electrode of the second thin-film transistor and a gate electrode of the first thin-film transistor may comprise a common material.
The peripheral-driving-circuit section may comprise, in addition to the above-mentioned first thin-film transistor, at least one of a top-gate, bottom-gate or dual-gate thin-film transistor having a channel region of a polycrystalline or amorphous silicon layer and a gate region formed above or below the channel region, and may further comprise a diode, a resistor, a capacitor and an inductor, each comprising a single-crystal, polycrystalline or amorphous silicon layer.
Thin-film transistors in the peripheral driving circuit and/or the display section have a single-gate or a multi-gate configuration.
Preferably, when the n- or p-channel thin-film transistor in the peripheral-driving-circuit section and/or the display section is a dual-gate type, the upper or a lower-gate electrode is electrically opened or a given negative voltage for the n-channel type or a given positive voltage for the p-channel type is applied so that the dual-gate type thin-film transistors operate as bottom- or top-gate type thin-film transistors.
The thin-film transistor in the peripheral-driving-circuit section may be the first thin-film transistor of an n-channel, p-channel or complementary type, and the thin-film transistor in the display section may be an n-channel, p-channel or complementary type when the channel region is a single-crystal silicon layer, a polycrystalline silicon layer, or an amorphous silicon layer.
After the single-crystal silicon layer is deposited, an upper-gate section including a gate insulating film and a gate electrode may be formed on the single-crystal silicon layer, and the single-crystal silicon layer may be doped with a Group III or V impurity through the upper-gate section to form the channel region, the source region and the drain region.
When the second thin-film transistor is a bottom-gate type or a dual-gate type, a lower-gate electrode composed of a heat resistant material is provided below the channel region, and a gate insulating film is formed on the gate electrode to form a lower-gate section, and the second thin-film transistor is formed by the same steps including the step for forming the step difference as those in the first thin-film transistor. In such a case the upper-gate electrode of the second thin-film transistor and the gate electrode of the first thin-film transistor may be composed of a common material.
The single-crystal silicon layer formed on the lower-gate section may be doped with a Group III or V impurity to form a source region and a drain region, followed by an activation treatment.
The source and drain regions of the second thin-film transistor may be formed by ion-implantation of the above impurity on the single-crystal silicon layer through a resist mask and may be subjected to activation treatment, and the gate electrode of the second thin-film transistor may be formed after the formation of the gate insulating film.
The method also may be such that, when the second thin-film transistor is of the top-gate type, the source and drain regions of the first and second thin-film transistors are formed by ion implantation of the above-mentioned impurities through a mask constituted by a resist, followed by an activation treatment, and thereafter a gate portion composed of a gate insulating film and the gate electrode of the second thin-film transistor is formed.
The method also may be such that, when the second thin-film transistor is of the top-gate type, a gate portion of the second thin-film transistor, composed of a gate insulating film and a gate electrode made of a heat-resistant material, is formed after the deposition of the single-crystal silicon layer, and the source and drain regions of the first and second thin-film transistors are formed by ion implantation of the above-mentioned impurities through a mask constituted by the above-mentioned gate portion and a resist, followed by an activation treatment.
The resist mask used when the LDD structure was formed may be left unremoved and the ion implantation for forming the source and drain regions may be performed through a resist mask which covers the remaining mask.
The substrate may be optically opaque or transparent, and may be provided with pixel electrodes for a reflective or transmissive display.
The display section may have a lamination configuration of the pixel electrodes and a color filter layer which may be-formed on a display array, whereby the aperture ratio and the luminance are improved and costs can be decreased due to omission of a color filter substrate and improved productivity.
When the pixel electrodes are reflective electrodes, unevenness is preferably imparted to a resin film so that the resin film has optimized reflective characteristics and viewing-angle characteristics, and then the pixel electrodes are formed, whereas, when the pixel electrodes are transparent electrodes, the surface is preferably planarized by a transparent planarization film and then the pixel electrodes are formed on the planarized plane.
The display section is illuminated or dimmed by being driven by the above-described MOSTFT: for instance, the display section may comprise, for example, a liquid crystal display (LCD), an electroluminescent (EL) display, a field emission display (FED), a light-emitting polymer display (LEPD), or a light-emitting diode (LED) display. In this case, the arrangement may be such that a plurality of pixel electrodes are arranged in a matrix in the display section and a switching device is connected to each pixel electrode.
In accordance with a fourth aspect of the present invention, there is provided a method of producing an electrooptical device, as well as a method of producing a driving substrate for such an electrooptical device, having a first substrate carrying a display section provided with pixel electrodes and a peripheral-driving-circuit section provided on a periphery of the display section, a second substrate, and an optical material disposed between the first substrate and the second substrate; the method comprising the steps of: a gate-forming step for forming a gate portion including a gate electrode and a gate insulating film on one face of the first substrate; a step-forming step for forming a step difference on the one face of the first substrate; a layer-forming step for forming a melt layer of a low-melting-point metal containing silicon on the first substrate having the gate portion and the step difference; a deposition step for depositing a single-crystal silicon layer by allowing the silicon of the melt layer to grow by graphoepitaxy by a cooling treatment using as a seed the step difference on the substrate; a step for effecting a predetermined treatment on the single-crystal silicon layer, thereby forming a channel region, a source region and a drain region; and a step for forming a first thin-film transistor of bottom-gate type having the gate portions on the below the channel region and constituting at least part of the peripheral-driving-circuit section. The above-mentioned thin-film transistor may be a field effect transistor (FET) or a bipolar transistor. The FET, when used, may be a MOS-type FET or a junction-type FET.
In accordance with this aspect of the present invention, the same advantages as those offered by the first aspect of the invention are obtainable, except for the advantage derived in the first aspect from the use of the dual-gate structure of the thin-film transistor. In addition, this aspect offers the following advantage by virtue of the-use of the melt layer of silicon-containing low-melting-point metal.
Namely, the melt of the low-melting-point metal can be prepared at a comparatively low temperature of, for example, 350xc2x0 C., and can easily be applied to the substrate having the step differences serving as the seed, provided that the substrate has been heated to a temperature slightly above the temperature at which the melt is prepared. Thus, a single-crystal silicon layer can uniformly be formed at a comparatively low temperature of from, for example, 350 to 400xc2x0 C.
In the graphoepitaxy in used in this aspect of the present invention, a single-crystal silicon thin-film having a variety of P-type impurity concentration and high mobility can be readily produced by adjusting the composition ratio of the melt of the melting-point metal, the temperature of the melt, the heating temperature of the substrate, and the cooling rate, thus enabling easy control of the threshold voltage (Vth) of the device, which in turn enables the device to operate at a high speed due to reduced resistance.
The melt layer of the silicon-containing low-melting-point metal may be doped with an adequate amount of Group III or V impurity, such as boron, phosphorus, antimony, arsenic, bismuth or aluminum, so that the type and the concentration of the impurity in the epitaxial single-crystal silicon, that is, the type (P-type or N-type) and the carrier concentration, are controllable.
Other features of the fourth aspect are substantially the same as those of the first, second and third aspects of the present invention.